Generally, a SpO2 sensor is used to determine a saturation level of oxygen in blood, by means of a light emitting portion and a light receiving portion thereof which are to be fixed to a suitable position of a user, such as fingers, during the whole determination process. At this point, the SpO2 sensor is usually used in connection with an accessory which can hold the light emitting portion and the corresponding light receiving portion and also could secure said portions to the desired position. A binding strap may be the most commonly used form of the accessory, which may be provided with mounting formations capable of holding the light emitting portion and the light receiving portion of the SpO2 sensor therein, and which could be wound around the application position together with the light emitting portion and the light receiving portion. However, the conventional binding strap has a shortcoming in that the binding strap is liable to be lost due to carelessness of the user when the binding strap is temporarily not in use and meanwhile the light emitting portion and the light receiving portion are detached from respective mounting formations. In addition, fixing the binding strap onto a desired position is usually performed by winding an elongated wrapping body of the binding strap around a U-shaped main body of the binding strap. Unfortunately, the current mounting manner usually results in looseness of the binding strap.